Field of the Invention
The present invention particularly relates to a favorable three-dimensional coordinate measuring apparatus and a favorable three-dimensional coordinate measuring method used to measure the shape of an object that is made up of various materials.
Description of the Related Art
It is widely known that a conventional three-dimensional coordinate measuring apparatus obtains three-dimensional coordinates on the surface of a measuring target by using the triangulation principle on an image obtained by a projection unit such as a projector projecting a stripe pattern, which is typified by a spatial coding method (such as Grey code) and the like, onto the measuring target, and capturing the light that is reflected by the measuring target with an imaging unit. In such an apparatus, there has been an issue in which the degree of precision with which three-dimensional coordinates are measured largely depends on the material of the measuring target.
In the case in which the measuring target is made up of a white colored material or the like whose surface diffusely reflects light and has a high reflectivity, such an apparatus can measure the three-dimensional shape with a high degree of precision and stability. However, the field of industrial production often sees the use of materials that not only reflect light off of their surface, but also allow light to penetrate and scatter internally.
Issues such as a decrease in the degree of measuring precision and measuring being impossible in the first place have occurred due to a phenomenon generally called sub-surface scattering or internal scattering that occurs in the measuring target object when measuring the target object with the conventional three-dimensional coordinate measuring apparatus. For this reason, when measuring such a target object, a step such as applying a white-colored powder or the like to the surface of the target object in advance is required. As a result, this has become an obstacle that largely restricts the scope of application of the three-dimensional coordinate measuring apparatus. Thus, measuring apparatuses that perform measuring using various methods have been proposed. Typical examples are disclosed in Japanese Patent No. 5202012 (hereinafter Patent Document 1), Japanese Patent Laid-Open No. 2012-251893 (hereinafter Patent Document 2), Japanese Patent No. 2517062 (hereinafter Patent Document 3), and Robust 3-D Shape Measurement by Modulated Slit Light Against Interreflection and Subsurface Scattering: Furuse. T, Hiura. S, Sato. K, Meeting on Image Recognition and Understanding (MIRU) 2009 (hereinafter, Non-Patent Document 1).
However, in the method described in Patent 1, a peak position is estimated based on the asymmetry of a waveform, but no consideration is made for internal scattering when measuring a translucent object, and thus the effect of correction is limited. Also, in the method described in Patent Document 2, the waveform of a luminance pattern depends not only on the material of the measuring target object, but on many parameters such as the shape of the target object, the geometric relationship between the light source and the light receiving unit, and the like. For this reason, in order to measure a broad range of target objects, a need arose to obtain, in advance, a vast number of reference luminance patterns in order to cover various combinations.
Furthermore, in the method described in Patent Document 3, three-dimensional measuring can be performed at high-speed, but no countermeasures are taken for measuring a translucent object that causes internal scattering, and thus increasing the degree of precision is limited. In the method in Non-Patent Document 1, a long time is required for measuring as there is a need to project many pattern lights and perform imaging in order to measure the entire target object.